1. Field of the Invention
Aspects of the present invention relate to a catalytic system for removing carbon monoxide (CO), and more particularly, to a low-temperature, cost-effective, high-efficiency catalytic system for removal of CO in reformed fuel for use in fuel cells.
2. Description of the Related Art
Fuel cells are electricity generation systems that directly convert the chemical energy of oxygen, and hydrogen in hydrocarbons, such as methanol, ethanol, and natural gas, to electrical energy.
Fuel cell systems typically include a fuel cell stack, a fuel processor (FP), a fuel tank, and a fuel pump. The fuel cell stack is the main body of the fuel cell, and is a stack of a plurality (several to several tens) of unit cells, each including a membrane electrode assembly (MEA) and a separator (or bipolar plate).
The fuel pump supplies fuel from the fuel tank to the fuel processor. The fuel processor produces hydrogen by reforming and purifying the fuel and supplies the hydrogen to the fuel cell stack. The fuel cell stack receives the hydrogen and generates electrical energy by electrochemical reaction of the hydrogen with oxygen.
A reformer of the fuel processor reforms hydrocarbon fuel using a reforming catalyst. A hydrocarbon fuel typically contains sulfur compounds. Since the reforming catalyst is easily poisoned by sulfur compounds, it is necessary to remove the sulfur compounds prior to reforming hydrocarbon fuel. Thus, hydrocarbon fuel is subjected to desulfurization prior to a reforming process (see FIG. 1).
Hydrocarbon reforming produces carbon dioxide (CO2) and a small quantity of carbon monoxide (CO), together with hydrogen. Since CO acts as a catalyst poison in electrodes of the fuel cell stack, reformed fuel cannot be directly supplied to the fuel cell stack. Thus, a CO removal process is needed. At this time, it is preferable to reduce the CO levels to less than 10 ppm.
CO can be removed by the high-temperature shift reaction represented by Reaction Scheme 1 below:CO+H2O→CO2+H2  <Reaction Scheme 1>
The high-temperature shift reaction is performed at a high temperature of 400 to 500° C. Thus, the high-temperature shift reaction requires much additional equipment and is inefficient in energy utilization. Furthermore, there arises a serious problem in that a methanation reaction occurs between CO to be removed and reformed hydrogen, as represented by Reaction Scheme 2 below:CO+3H2→CH4+H2O  <Reaction Scheme 2>
The high-temperature shift reaction can be followed by a low-temperature shift reaction at a temperature of 200 to 300° C. However, even after performing both of these reactions, it is very difficult to reduce the CO levels to less than 5,000 ppm.
To solve this problem, a preferential oxidation reaction (so-called “PROX” reaction) represented by Reaction Scheme 3 below can be used:CO+½O2→CO2  <Reaction Scheme 3>
However, the PROX reaction has the disadvantages that the reaction rate is too slow at low temperatures, and that a reverse water gas shift reaction occurs at high temperatures.
In addition, the above-described reactions act as rate-limiting steps of the overall reaction in a fuel cell system due to the slow reaction rate, and require a large quantity of water and thus much additional equipment, which makes it difficult to use a fuel cell as a power source when compactness and mobility are desired, such as in automobiles, etc.
Meanwhile, in view of the above problems, an attempt has been made to remove CO in fuel using a gold (Au) catalyst and polyoxometalate (Kim, W. B., et al., “Powering fuel cells with CO via aqueous polyoxometalates and gold catalysts,” Science 305, 2004, pp. 1280-1283).
In this catalytic system, CO is converted to CO2 and polyoxometalate is reduced by a contact reaction of liquid water with CO over a gold nanotube catalyst. The reduced polyoxometalate is reoxidized in an anode of a fuel cell. The reoxidized polyoxometalate is recycled, and electrons produced during the oxidation of CO travel to an external load.
This catalytic system has an advantage that electricity is directly generated by direct supply of the electrons produced during the oxidation of CO to the external load. However, there are disadvantages in that the catalytic system is structurally complicated by the recycling system, the preparation of the gold nanotube catalyst using no support increases catalyst costs, and a water supply to a fuel processor is separately required.
Therefore, it is necessary to develop a simple, low-cost, high-efficiency fuel processor that offers a simple system construction and low catalyst costs, and does not require a separate water supply.